Study of transpiring fluid dynamics in supercritical water oxidation using a transparent reactor

The transpiring wall reactor (TWR) is considered to be one of the most promising reactors because it minimizes both corrosion and salt precipitation problems that seriously hinder the industrialization of supercritical water oxidation technologies. A transparent reactor is built to study the fluid dynamics of transpiring flow, which are the foundation of reactor design and optimization. The results showed that the transpiring flow is anisotropic with respect to the surface of the transpiring wall due to both the static pressure and viscous resistance. Finally, the novel idea of using air as the transpiring fluid instead of water is presented in an attempt to alleviate current TWR problems such as high energy consumption, high volume of pure water consumption, and temperature fluctuation in the reaction area. A series of experiments and theoretical derivations demonstrate that this novel idea is feasible.     

A novel concept reactor design for preventing salt deposition in supercritical water

Reactor plugging and corrosion are the key problems which hinder commercial applications of supercritical water oxidation and gasification, and can be efficiently overcome by preventing salt deposition on internal surface of reactor. In this work the problems caused by salt deposition and the correspondingly main solutions are further reviewed objectively. A novel reactor is designed and manufactured with a feed rate of about 100 L/h for sewage sludge treatment. The reactor combines the characteristics of Modar reactor and transpiring wall reactor for the first time, which is expected to prevent reactor plugging and corrosion as well as to decrease catalyst deactivation rate. The reactor is the core equipment of the first pilot-scale plant for supercritical water oxidation in China. Further optimizations of reactor configuration and operational parameters need plenty of experiments and/or a long-time test with sewage sludge in the subsequent work.     

Analysis of the scale up of a transpiring wall reactor with a hydrothermal flame as a heat source for the supercritical water oxidation

Experimental data from a tubular reactor and from a transpiring wall reactor (TWR) are used to analyze the scaling up of SCWO reactors operating with a hydrothermal flame as a heat source. Results obtained with the tubular reactor show that fluid velocity inside the reactor determines the minimum injection temperature at which a stable hydrothermal flame is formed. When the fluid velocity inside of the reactor is lower, the extinction temperature of the hydrothermal flame in that reactor is also lower. Using this reactor, extinction temperatures are always near or above the critical temperature of water. Total TOC removals are possible working with isopropyl-alcohol at temperatures between 650 and 700 °C and residence times of 0.5 s. Results of the TWR show that steady operation with a hydrothermal flame inside is possible even when reagents are injected at subcritical conditions as low as 170 °C. Temperature measurements show that reaction is not initiated in the injector but in the reaction chamber, where fluid velocity is lower than 0.1 s. This was explained by estimating that the flame front velocity of a hydrothermal flame is of the order of 0.1 m/s. Thus, it is expected that the flame is stabilized in the reaction chamber and not in the injector, where fluid velocities are higher than 2 m/s. A previously developed model of the TWR was modified in order to describe the ignition in the reaction chamber and not in the injector. The model reproduces satisfactorily experimental data and it was used to propose the design of scaled up reactors for SCWO with a hydrothermal flame inside.     

Study on the uniformity of water film in a transpiring wall reactor for supercritical water oxidation

A three-dimensional computational fluid dynamic model of a transpiring wall reactor for supercritical water oxidation has been built to optimize the uniformity of water film. Results show that the temperature and species distributions at the nozzle outlet deviate from the reactor centre. The inner wall of the porous tube near the transpiring water injection tube displays low temperatures, while high temperatures are recorded far from the injection tube. The circumferential temperature distribution on the inner wall of the porous tube is uneven. This phenomenon is due to the uneven injection of the transpiring water, leading to the uneven protection of the water film and local overheating of the porous wall. The injection velocity of the transpiring water significantly decreases when the number of injection tubes is increased, and the circumferential velocity and temperature distributions on the porous wall gradually become even. Moreover, high pressure drops across the porous wall at low porosities are useful for the uniform injection of the transpiring water. This characteristic is also conducive to obtaining a more uniform water film protection.     

Hydrothermal flames in supercritical water oxidation: investigation in a pilot scale continuous reactor

Hydrothermal flames at 25 MPa supercritical water environment were investigated using a 4800 ml reaction tower, in which the sapphire windows were fitted for optical access. Down flowing hydrothermal flames were observed for oxidation of 2-propanol when the reactor was fed with inlet organic concentration higher than 2 vol% and air ratio higher than 1.8. Flame temperature, as high as 1100 °C, was measured by means of a thermocouple and the temperature was found to be strongly influenced by air ratio. Effective and stable oxidation of organics with TOC removal rate of 99.9% was achieved. Dioxins were also decomposed with a ratio higher than 99.9%, within 1 min reaction time in this reactor configuration.     

Corrosion control methods in supercritical water oxidation and gasification processes

Use of supercritical water (SCW) as a medium for oxidation reactions, conversion of organic materials to gaseous or liquid products, and for organic and inorganic synthesis processes, has been the subject of extensive research, development, and some commercial activity for over 25 years. A key aspect of the technology concerns the identification of materials, component designs, and operating techniques suitable for handling the moderately high temperatures and pressures and aggressive environments present in many SCW processes. Depending upon the particular application, or upon the particular location within a single process, the SCW process environment may be oxidizing, reducing, acidic, basic, nonionic, or highly ionic. Thus, it is difficult to find any one material or design that can withstand the effects of all feed types under all conditions. Nevertheless, several approaches have been developed to allow successful continuous processing with sufficient corrosion resistance for an acceptable period of time. The present paper reviews the experience to date for methods of corrosion control in the two most prevalent SCW processing applications: supercritical water oxidation (SCWO) and supercritical water gasification (SCWG).     

Ion exchange resins destruction in a stirred supercritical water oxidation reactor

Spent ion exchange resins (IERs) are radioactive process wastes for which there is no satisfactory industrial treatment. Supercritical water oxidation offers a viable treatment alternative to destroy the organic structure of resins, used to remove radioactivity. Up to now, studies carried out in supercritical water for IER destruction showed that degradation rates higher than 99% are difficult to obtain even using a catalyst or a large oxidant excess. In this study, a co-fuel, isopropanol, has been used in order to improve degradation rates by initiating the oxidation reaction and increasing temperature of the reaction medium. Concentrations up to 20 wt% were tested for anionic and cationic resins. Total organic carbon reduction rates higher than 99% were obtained from this process, without the use of a catalyst. The influence of operating parameters such as IERs feed concentration, nature and counterions of exchanged IERs were also studied.     

Non-catalytic Oppenauer oxidation of alcohols in supercritical water

Non-catalytic Oppenauer oxidation was applied for alcohols, such as benzyl alcohol (4) and benzhydrol (1), in the presence of an excess amount of carbonyl compound, formaldehyde (5a), as an oxidant with and without water. Oppenauer oxidation took place in both reactions of 4 and 1 to afford the oxidation products, benzaldehyde (6) (95%) and benzophenone (2) (64%), concomitant with relatively small amounts of reduction products, toluene (7) (1%) and diphenylmethane (3) (13%), respectively, at 400 °C for 10 min without water in an SUS 316 batch-type tubular reactor. Lower yields of oxidation products 6 (68%) and 2 (30%) were obtained in supercritical water under the conditions of 400 °C, 10 min, and 0.35 g/mL water density, while the formation of the reduction products 7 and 3 was completely suppressed. Thus, water was indispensable for the clean and highly selective Oppenauer oxidation of 4 and 1 to yield 6 and 2.     

超临界水氧化技术中盐沉积问题的研究进展

超临界水氧化技术在处理高浓度难降解有机废水时具有去除率高、反应速度快、无二次污染等独特的优势,但存在盐沉积引起的反应器堵塞问题。本文针对国内外盐沉积问题研究的技术现状进行系统综述,归纳了盐沉积问题的研究方法,总结了部分盐在超临界水中的溶解度以及沉积和分离特性,阐述了盐沉积理论及从源头控制盐沉积途径,介绍了避免盐沉积引起反应器堵塞的技术方法,并对后续的研究进行了展望。指出盐沉积问题的解决还需进一步研究盐形成和沉积机理,建立不同盐类混合物的相图,研究盐沉积动力学和多组分系统的相行为,考察多组分盐之间的相互作用机制。这些信息有利于研究人员掌握超临界水氧化技术中盐沉积问题的基础知识和发展方向,有助于在实际工程应用中指导反应器结构设计和优化运行条件。     

Co-oxidation of ammonia and isopropanol in supercritical water in a tubular reactor

Improvements in the ammonia removal were only appreciated with the lowest IPA/NH₃ molar ratios (0.125 and 0.25) while the oxygen ratio did not have significant influence in the ammonia removal. Nevertheless, a direct relation between the nitrate concentration and the oxygen in excess was found. Nitrate concentration was also found to increase when the IPA/ammonia ratio increased.     

Catalytic denitrogenation of hydrocarbons through partial oxidation in supercritical water

In this work, the denitrogenation of hydrocarbons under supercritical water oxidation environment was investigated in a rotated bomb reactor at 623-723 K and 25-35 MPa over sulfided NiMo catalyst. Quinoline was used as a model nitrogen-containing compound. A high reduction of total nitrogen up to about 85% was obtained. The denitrogenation pathway is composed of two consecutive steps: in situ H₂ generation and the hydrogenation of quinoline. The hydrogenation mechanism of quinoline varies with reaction temperature because of the participation of supercritical water in HDN step. The strong adsorption of quinoline and its hydrogenation intermediates on catalyst surface has an adverse influence on total nitrogen reduction rate.     

Investigation of thermochemical conversion of biomass in supercritical water using a batch reactor

This study focused on gasification of biomass and a biomass model compound. Data are presented that show the presence of supercritical water enhances gasification efficiency, as it participates as both a solvent and a reactant. It is established that biomass gasification efficiencies are in the same range for all types of biomass. The thermodynamic changes of state are functions of elemental composition, not biomass species. The oxidation state of carbon atom of biomass is a key variable in determining the changes in enthalpy during both conventional combustion and supercritical water gasification. The oxidation state of the feed (together with the reaction conditions that influence the degree to which water participates as a reactant) also determines the vapor product composition.Decomposition reactions to vapor products are rapid and complete at high temperature (?550 °C), catalytic mediation is not required. Temperature and residence time are important operating parameters for SCW gasification. Less important are the pressure of gasification (in the range of 40-67 MPa) and the presence of catalyst. The vapor yield, gas composition, the carbon and hydrogen balance of SCW gasification are functions of gasification temperature, residence time and biomass load (concentration).     

A process for generating power from the oxidation of coal in supercritical water

A theoretical study of power generation from oxidation of coal by supercritical water oxidation (SCWO) is presented. Two versions of SCWO power plant are compared to two of the most efficient conventional power plant processes: pulverised coal power plants and pressurised fluidised bed power plant. The effects of steam pressure and temperature on produced (W_p), consumed (W_c) and net work (W_N) are calculated in order to compare the efficiency of these power plants for the same steam conditions. Enthalpies have been calculated using residual enthalpies by Peng-Robinson equation of state. Calculated results show that net work in SCWO power plant is 5% higher than in other power plants, due to the fact that no air surplus is necessary for complete combustion and because steam is produced by direct heating. Energetic efficiency of SCWO increases more quickly with temperature than for the other power plants. The effect of steam pressure is different: until 30 MPa power plant efficiencies increase more quickly in SCWO power plants than in conventional plants, but when steam pressures increases beyond 30 MPa, efficiencies decrease in SCWO power plants.     

Effects of structural parameters on water film properties of transpiring wall reactor

Corrosion and salt deposition problems severely restrict the industrialization of supercritical water oxidation. Transpiring wall reactor can effectively weaken these two problems by a protective water film. In this work, methanol was selected as organic matter, and the influences of vital structural parameters on water film properties and organic matter removal were studied via numerical simulation. The results indicate that higher than 99% of methanol conversion could be obtained and hardly affected by transpiration water layer, transpiring wall porosity and inner diameter. Increasing layer and porosity reduced reactor center temperature, but inner diameter's influence was lower relatively. Water film temperature reduced but coverage rate raised as layer, porosity, and inner diameter increased. Notably, the whole reactor was in supercritical state and coverage rate was only approximately 85% in the case of one layer. Increasing reactor length affected slightly the volume of the upper supercritical zone but enlarged the subcritical zone.     

Fast catalytic oxidation of phenol in supercritical water

The catalytic oxidation of phenol in water over a commercial oxidation catalyst, CARULITE 150, was investigated in a fixed bed flow reactor at 250 atm and temperatures from 380°C to 430°C. The phenol and oxygen concentrations at the reactor entrance varied between 0.070 and 1.24 mmol/l, and 9.60 and 39.6 mmol/l, respectively. The reaction conditions produced phenol conversions and selectivities to CO₂ much higher than those produced by non-catalytic oxidation. The kinetics of phenol disappearance and of CO₂ formation were both roughly first-order, and the activation energies were 31 and 47 kcal/mol, respectively. The catalyst did not undergo continuous deactivation during the catalytic oxidation, but rather maintained a high activity even after several days of continuous operation.     

Phenol oxidation over CuO/Al2O3 in supercritical water

Phenol was oxidized in supercritical water at 380–450°C and 219–300 atm, using CuO/Al₂O₃ as a catalyst in a packed-bed flow reactor. The CuO catalyst has the desired effects of accelerating the phenol disappearance and CO₂ formation rates relative to non-catalytic supercritical water oxidation (SCWO). It also simultaneously reduced the yield of undesired phenol dimers at a given phenol conversion. The rates of phenol disappearance and CO₂ formation are sensitive to the phenol and O₂ concentrations, but insensitive to the water density. A dual-site Langmuir–Hinshelwood–Hougen–Watson rate law used previously for catalytic SCWO of phenol over other transition metal oxides and the Mars–van Krevelen rate law can correlate the catalytic kinetics for phenol disappearance over CuO. The supported CuO catalyst exhibited a higher activity, on a mass of catalyst basis, for phenol disappearance and CO₂ formation than did bulk MnO₂ or bulk TiO₂. The CuO catalyst had the lowest activity, however, when expressed on the basis of fresh catalyst surface area. The CuO catalyst exhibited some initial deactivation, but otherwise maintained its activity throughout 100 h of continuous use. Both Cu and Al were detected in the reactor effluent, however, which indicates the dissolution or erosion of the catalyst at reaction conditions.     

CFD simulation of a transpiring‐wall SCWO reactor: Formation and optimization of the water film

A two‐dimensional axisymmetric computational fluid dynamics model of a transpiring wall reactor for supercritical water oxidation was developed using the commercial software Fluent 6.3. Numerical model was validated by comparisons with experimental temperature profiles and product properties (total organic carbon and CO). Compared with the transpiration intensity, the transpiring water temperature was found to have a more significant influence on the reaction zone. An assumption that an ideal corrosion and salt deposition inhibitive water film can be formed when the temperature of the inner surface of the porous tube is less than 374°C was made. It was observed that lowering transpiring water temperature is conducive to the formation of the water film at the expense of feed degradation. The appropriate mass flux ratio between the total transpiring flow and the core flow was determined at 0.05 based on the formation of the water film and feed degradation. © 2015 American Institute of Chemical Engineers AIChE J, 62: 195–206, 2016     

Upgrading of asphalt with and without partial oxidation in supercritical water

Supercritical water and supercritical water partial oxidation treatments were applied to the upgrading of asphalt. Asphalt was converted at 613-673 K, 0-0.5 g/cm³ water density under argon or air atmosphere. Under an argon atmosphere and 0.5 g/cm³ water density, both the asphaltene conversion and desulfurization increased with increasing temperature. At 673 K, the asphaltene conversion and the yield of CO₂ increased with an increasing water density. Water apparently participated in the reaction and its hydrogen was used for capping the free radicals generated during the upgrading of asphalt resulting in an increased yield of maltene. Under an air atmosphere at 673 K, asphaltene conversion was lower but desulfurization was higher than those obtained in an argon atmosphere.     

超临界水氧化技术研究及进展

介绍了超临界水的特性 ,并对超临界水氧化技术的研究及应用进行了综述 ,提出了超临界水氧化技术的工程应用开发中存在的问题     

Decomposition of 2-chlorophenol by supercritical water oxidation with zirconium corrosion

The 2-chlorophenol (2-CP) was oxidized in a continuous anti-corrosive supercritical water system. The variation of decomposition efficiency by the corrosion of zirconium 702 was also studied at the variation of feed concentration and reaction time. According to AES depth profile, the oxygen penetration depth to zirconium was not proportional to the exposure time. It might stem from the formation of zirconium oxide layer on the surface delaying the corrosion. However, the increase in feed concentration accelerated the corrosion of zirconium. The corrosion of zirconium at low feed concentration led to the improvement of decomposition efficiency due to the catalytic effect of formed zirconium oxides, while that at high feed concentration deteriorated the decomposition efficiency owing to large consumption of oxidant in corrosion.